Neurodegeneration due to neurodegenerative brain disease and injury is a major health and economic concern. Current models predict at least a more than threefold rise in the total number of persons with Alzheimer's disease between 2000 and 2050. A well accepted concept for a basic cause for neurodegeneration is overexcitation by glutamate as well as impairments of mitochondria leading to death of nerve cells in the brain (Camins et al. Methods Find Exp Clin Pharmacol. 2008; 30:43-65; Nakamura et al. Apoptosis. 2010; 15:1354-63). Potential treatments have been designed and tested, for example, by inhibition of glutamate receptors. However, such approaches have not matured to effective treatments, probably because the causes for neurodegeneration are not well understood (Byrnes et al. Neurotherapeutics. 2009; 6:94-107). Alternative approaches have therefore been explored, for example by preventing mitochondria-based cell death processes (Veenman et al. Drug Dev Res. 2000; 50:355-370; Veenman et al. Neurochem. 2002; 80:917-27; Mattson et al. Trends Mol Med. 2003; 9:196-205). Furthermore, glia constitutes the vast majority of brain cells, and serves to maintain and protect healthy neurons. Indeed it is now well realized that glia presents a critical factor in the progress of neurodegeneration (Rossi et al. Brain Res Bull. 2009; 80:224-32). Thus, glia should be one of the factors to be included for the development of effective treatments for neurodegeneration.
Traumatic brain injury (TBI) is characterized by sudden physical damage to the brain. It is caused by many factors including warfare, terrorism, motor vehicle accidents and other traffic accidents, work related accidents, sports injuries, violent crimes, household accidents, child abuse and domestic violence, or by an object passing through the skull, for example gunshot wounds, etc. The physical, behavioral, and/or mental changes that may result from TBI depend on the areas of the brain that are injured. Injuries include focal and diffuse brain damage. Focal damage is confined to a small area of the brain. The focal damage is most often at the point where the head hits an object or where an object, such as a bullet, enters the brain. Diffuse damage is spread throughout the brain. While treatment of head wounds immediately after injury has improved steadily in the past few decades, lingering effects such as disabilities are still likely after moderate and severe TBI (Kluger et al. J Am Coll Surg. 2004; 199:875-79). It is now well understood that the primary injury of TBI is followed for hours and days by a process of secondary brain injury (Gaetz et al. Clin Neurophysiol. 2004; 115:4-18; Sullivan et al. J Neurosci Res. 2005; 79:231-39). Major factors contributing to this second wave of brain damage include: excitatory amino acids such as glutamate, Ca++ homeostasis, and reactive oxygen species (ROS) (Kluger et al. 2004, Gaetz et al. 2004). Mitochondria are one of the cell organelles affected by secondary brain injury and in particular collapse of the mitochondrial membrane potential (ΔΨm) appears to take a central role among the factors leading to neuronal cell death due to secondary brain injury and neurodegenerative diseases (Kluger et al. 2004; Gaetz et al. 2004). This suggests that targeting the regulation of the ΔΨm, immediately after sustained brain injury, would have significant therapeutic implications for TBI. As the TSPO is a major component for these mechanisms, targeting the TSPO to treat secondary brain injury, neurodegenerative diseases, and related conditions represents a novel and potentially very effective therapeutic approach.
Importantly, the TSPO has been associated with the voltage dependent anion channel (VDAC), located in the outer mitochondrial membrane, which functions as a major channel allowing passage of small molecules and ions between the mitochondrial inter-membrane space and cytoplasm (McEnery et al. Proc Natl Acad Sci USA. 1992; 89:3170-4; Veenman et al. Anticancer Agents Med Chem. 2014; 14:559-77). Opening of the VDAC can contribute to collapse of the ΔΨm. The TSPO has also been associated the adenine nucleotide translocator (ANT), which is located in the inner mitochondrial membrane, and can form a lethal pore also contributing to collapse of the ΔΨm. The discovery of TSPO's essential role in programmed cell death processes, including collapse of the ΔΨm, related to neurodegeneration, as it can occur due to disease and brain trauma, has exemplified the TSPO as a target for drug development for neurodegenerative diseases and treatment of brain trauma. In this context, TSPO was found to be involved in the generation of ROS that take part in the induction of the mitochondrial apoptosis cascade, as well as other forms of programmed cell death. These ROS are known to cause detachment of cytochrome c from cardiolipins located at the inner mitochondrial membrane. In addition, ROS contribute to pore opening in the outer mitochondrial membrane allowing release of cytochrome c into the cytosol. This forms the initiating step for activation of the mitochondrial apoptosis pathway (Veenman et al. J Bioenerg Biomembr. 2008; 40:199-205; Veenman et al. 2014). These data provide an understanding regarding the mechanisms whereby TSPO may serve as a target to modulate programmed cell death rates. This has implications for drug design to treat diseases such as neurodegeneration and the effects of brain trauma, and other forms of brain damage, in particular involving cell death in the brain.
TSPO was first detected by its capability to bind benzodiazepines in peripheral tissues and later was detected also in glial cells in the brain, hence its previous name peripheral benzodiazepine receptor (PBR) (Papadopoulos et al. Trends Pharmacol Sci. 2006; 27:402-9; Veenman and Gavish. Pharmacol Ther. 2006; 110:503-24; Veenman et al. Curr Pharm Design. 2007; 13:2385-2405). It is known that this mitochondrial protein can be pharmacologically regulated by specific ligands (Veenman et al. J Neurochem. 2002; 80:917-27; Veenman and Gavish. Pharmacol Ther. 2006; 110:503-24; Veenman et al. 2007). Typical examples of classical TSPO ligands are PK 11195 and Ro5 4864 (PK 11195 is an isoquinoline derivative and Ro5 4864 is a benzodiazepine). TSPO functions include: regulation of cell death processes, regulation of the cell cycle, regulation of gene expression, modulation of steroid production, involvement in cell migration, involvement in cell differentiation, involvement in angiogenesis, involvement in excitotoxic cell death, and involvement in inflammation and immune responses. The expression of TSPO is enhanced during neurodegeneration occurring, e.g., in Alzheimer, Parkinson, and Huntington disease, and also due to brain trauma (Gavish et al. Pharmacol Rev. 1999; 51:629-50; Veenman and Gavish. 2000; 50:355-70; Veenman and Gavish. 2006; Veenman and Gavish. Curr Mol Med. 2012; 12:398-412; Veenman et al. Pharmacogenet Genomics. 2012; 22:606-19; Veiga et al. Glia. 2007; 55:1426-36; Papadopoulos et al. Exp Neurol. 2009; 219:53-7). In the CNS, the primary cellular location of TSPO is astrocytes and microglia. However, TSPO can also be expressed in neurons that are on a cell death pathway. Silencing the TSPO by genetic manipulation (siRNA and antisense RNA) prevents programmed cell death, including cell death induced by glutamate. Interestingly, TSPO levels are increased in U118MG cells exposed to lethal levels of glutamate. U118MG cells are of astrocytic origin. For at least these reasons, the TSPO presents a promising target to protect the brain from neuropathological processes (Veiga et al. 2007; Panickar et al. Glia. 2007; 55:1720-7). Apart from its role in cell death, other elucidated functions under the control of the TSPO, such as regulation of gene expression, involvement in inflammation and immune response, involvement in cell migration, cell differentiation, involvement in angiogenesis, and its specific involvement in excitotoxic cell death, emphasize the importance of the TSPO as a target for treatment against cell death in the brain due to disease and brain trauma, including its contribution to repair and restorative mechanisms (Bode et al. Pharmacogenet Genomics. 2012; 22:538-50; Veenman et al. Pharmacogenet Genomics. 2012). It has previously been found that the classical TSPO ligands PK 11195 and Ro5 4864 present moderate neuroprotective effects in vivo (see, e.g., Veenman et al. 2002; Veiga et al. 2005; 80:129-37; Veenman Gavish. 2012). The effects of classical TSPO ligands in vitro approximate those found by silencing TSPO by genetic manipulation (Veenman et al. Biochem Pharmacol. 2004; 68:689-98; Kugler et al. Cell Oncol. 2008; 30:435-50; Veenman et al. 2008; Levin et al. Biochemistry. 2005; 44:9924-35; Zeno et al. Biochemistry. 2009; 48:4652-61).
U.S. Pat. No. 8,541,428 discloses phthalazine, quinazoline, and quinoxaline derivatives, pharmaceutical compositions containing the compounds, and their therapeutic use in treating and preventing brain damage resulting from traumatic brain injuries (TBI), and in treating and preventing neurodegenerative diseases. Compounds disclosed in said patent publication were shown to bind to the PBR (TSPO) with varying affinity and were shown to reduce cell death induced by glutamate in cell culture. Furthermore, cell protection conferred by these compounds occurred with neuronal type cells (SH SY 5Y) as well as glial cell types (U118MG).
U.S. Pat. No. 6,765,006 discloses quinazolines and other heterocycles which are antagonists or positive modulators of AMPA receptors, and the use thereof for treating, preventing or ameliorating neuronal loss or treating or ameliorating neurodegenerative diseases.
A need in the art exists to develop improved agents that bind effectively to the TSPO, combined with the effect of preventing brain cell death, including various forms of programmed cell death (apoptosis, necrosis, autophagic cell death), as it occurs due to TBI and secondary brain damage and/or neurodegenerative diseases. Furthermore, for successful treatment of TBI, secondary brain damage, and neurodegenerative disease it is further desired that repair mechanisms are activated and/or facilitated.